1. Field of the Invention
The present invention relates to a braking control for a motor vehicle. More particularly, it relates to a braking force control system for a motor vehicle wherein a hydraulic pressure generated by the master cylinder (or a pump) of a braking system is controlled to increase or decrease, in accordance with drive conditions of the motor vehicle.
2. Description of the Prior Art
Heretofore, antiskid systems have been developed in order to secure the stability and steerability of a motor vehicle in the sudden braking operation, etc. thereof. With such a system, a hydraulic pressure generated by the master cylinder of a braking system is controlled to increase or decrease, in accordance with the drive conditions of the motor vehicle, e. g., the rotating situation of wheels and the situation of a road surface. Thus, the antiskid system prevents the wheels from undergoing any excessive braking force and from coming to lock.
According to Japanese Patent Application Laid-open No. 502423/1993, the target (or desired) braking hydraulic pressure of the braking system is obtained by a PI (proportional-plus-integral) feedback control based on the deviation between a target slip and an actual slip which is evaluated from wheel speeds, a vehicle speed, etc. Besides, the switching of an antiskid-hydraulic-pressure control valve is controlled in such a way that the switching time of the valve is evaluated on the basis of a reverse hydraulic pressure model according to the target braking hydraulic pressure mentioned above, a feed pressure which is supplied by the master cylinder of the braking system, and a side force which acts on the wheel.
In any of the braking force control systems of this type in the prior art, including the aforecited system of Japanese Patent Application Laid-open No. 502423/1993, the target braking hydraulic pressure serving as the index of the control is obtained directly from such parameters as the target slip, the wheel speeds and the vehicle speed. This has posed the problem that a finer comprehensive or overall control conforming to the motion characteristics of the motor vehicle cannot be realized.
By way of example, in evaluating the switching time of the valve by the use of the reverse hydraulic pressure model, the control system disclosed in Japanese Patent Application Laid-open No. 502423/1993 supposes a mere orifice as a valve model and calculates the valve switching time in accordance with the static model. In the practical hydraulic system, however, a high precision cannot be secured unless the change of a piping rigidity is considered. Further, the generated hydraulic pressure cannot be controlled at a satisfactory precision unless the valve switching time is calculated in accordance with a dynamic model in which even the motions of the motor vehicle, the gain and phase of an actuator, the hydraulic pressure characteristics of the actuator, etc. are taken into consideration.
Any of the prior-art control systems has therefore had the problem that the gain and phase of the actuator or the hydraulic pressure characteristics thereof, for example, sometimes fail to appropriately match with the actual motions of the motor vehicle, so the control of braking forces does not always become optimal in the vehicle behavior control.
In other words, the method stated before, in which the "target braking hydraulic pressure" is evaluated directly from the parameters of the target slip, the wheel speeds, the vehicle speed or the likes, has had the problem that, even if such target braking hydraulic pressures are favorably calculated for the right and left wheels independently of each other, by way of example, the "resulting actual braking forces to the right and left wheels" does not always match favorably with the real behavior state of the motor vehicle.
As a matter of fact, therefore, it has been actually difficult to positively utilize the control of the braking forces for the motion control, attitude control, etc. of the motor vehicle.